1. Field of the Invention
The present invention relates to a relief pressure switching apparatus for hydraulic working machine for switching a relief pressure in accordance with a specification of a working apparatus to be installed (such as a compression crushing apparatus and a breaker).
2. Description of the Related Art
For example, in a crusher for crushing a building or the like, there is a case where a working apparatus is selected and used from two types in accordance with a purpose.
FIGS. 4 and 5 exemplify the crusher formed by having a hydraulic excavator as a base.
The above crusher is formed by installing a working attachment 2 capable of raising, lowering, bending and stretching in a crawler traveling type base machine 1. At a front end of the working attachment 2, is attached a compression crushing apparatus 3 of an openable and closable type (FIG. 4) called as a nibbler, or a breaker 4 of an oscillating type (FIG. 5) as the working apparatus in accordance with contents of work. In the following, work performed by the compression crushing apparatus 3 is referred to as compression crushing work, and work performed by the breaker 4 is referred to as breaker work.
In this case, hydraulic actuators are different between the compression crushing apparatus 3 and the breaker 4, and oil supply and discharge routes for both the actuators are required to be different from each other. Therefore, there is a need for switching a circuit state of a hydraulic circuit in accordance with the working apparatus to be installed.
FIG. 6 shows the hydraulic circuit having a function of automatically switching the above circuit state.
The reference numeral 5 denotes a control valve of a hydraulic pilot switching type operated by a remote controlling valve 6 of a pedal operating type for example. The reference numeral 7 denotes a hydraulic pump serving as an actuator hydraulic source, and T denotes a tank. At the time of operating the control valve 5, oil from the hydraulic pump 7 is fed to a compression crushing cylinder 8 or a breaker cylinder 9 serving as the hydraulic actuator so as to operate the compression crushing cylinder 8 or the breaker cylinder 9.
Here, at the time of the compression crushing work, as well as a general cylinder circuit, both an inlet side and an outlet side of the compression crushing cylinder 8 are connected to the hydraulic pump 7 and the tank T through the control valve 5.
Meanwhile, at the time of the breaker work, when a back pressure is generated in a return line of the breaker cylinder 9 by a squeezing effect of the control valve 5, power of the breaker 4 in FIG. 5 is weakened and at worst the breaker 4 is stopped. Due to the above reason or the like, there is a need for directly connecting the return line of the breaker cylinder 9 to the tank T.
For switching the above circuit state, a switching valve 11 controlled by a controller 10 is provided.
The above switching valve 11 is formed by a main valve 12 of a hydraulic pilot switching type for switching between a compression crushing work position a and a breaker work position b, and an electromagnetic valve 13 for controlling the switching of the main valve 12 on the basis of an electric signal from the controller 10.
The electromagnetic valve 13 is provided with a compression crushing work position x for blocking a pilot pressure from a pilot hydraulic source 14 (also serving as a pilot hydraulic source for the remote controlling valve 6) to a pilot port 12a of the main valve 12 (for communicating the pilot port 12a to the tank T), and a breaker work position y for supplying the pilot pressure to the pilot port 12a. 
The above switching valve 11 works as in the following or the like.
In a state that the switch for changing work 15 is operated to the compression crushing work side, the electric signal is not sent from the controller 10 to the electromagnetic valve 13. Therefore, the electromagnetic valve 13 and the main valve 12 are at the compression crushing work positions x and a respectively as shown in the figure.
Consequently, there is a state that lines on the both sides of the compression crushing cylinder 8 are connected to the hydraulic pump 7 and the tank T respectively through the control valve 5.
Meanwhile, when the switch for changing work 15 is switched to the breaker work side, the electromagnetic valve 13 is switched to the breaker work position y by the electric signal from the controller 10. Therefore, the main valve 12 is also switched to the breaker work position b.
Consequently, the return line of the breaker cylinder 9 is directly connected to the tank T without passing through the control valve 5.
In FIG. 6, the reference numerals 16 and 17 denote relief valves for determining a maximum pressure of a hydraulic actuator circuit.
As mentioned above, the technique of automatically switching a circuit state by the switching valve in accordance with the type of the working apparatus is shown in Japanese Patent Laid-Open No. 2003-185042.
The compression crushing apparatus 3 and the breaker 4 have different flow rates and work pressures of the cylinders 8 and 9 from each other.
Therefore, a variable capacity type pump is used as the hydraulic pump 7, and tilting of the pump 7 is controlled by the signal from the controller 10 on the basis of the operation of the switch for changing work 15 so that a discharge amount of the pump is adjusted.
With regard to the work pressure, by using relief valves of a manually variable type (mechanical relief valves) as the relief valves 16 and 17, a set pressure is changed to a value determined in relation with a specified flow rate.
Meanwhile, a technique of automatically switching a relief pressure between high and low by a method of dividing two types between a breaker and other working apparatuses is shown in Japanese Patent Laid-Open No. 2002-294758.
As mentioned above, the technique of manually switching a circuit pressure in accordance with the type of the working apparatus to be installed, and the technique of automatically switching between high and low are publicly known respectively.
However, even with the same type of working apparatus, a flow rate thereof is different in accordance with a specification thereof, and a pressure thereof is different in accordance with the flow rate. Therefore, there is a need for variously switching the circuit pressure in accordance with not only the type of working apparatus but also the specification.
In this case, the publicly known technique of automatically switching is not capable of satisfying the above request. In the technique of manually switching, for every time the working apparatus is exchanged, the relief valves 16 and 17 are manually operated according to the specification or the like so as to adjust the set pressure again, and the switching operation is very troublesome.